Bicontinuous composites

ABSTRACT

A ceramic foam having pores in the range of 20 to 800 microns and 10 to 30 % of theoretical density is placed in a preheated mould and molten metal or plastics is drawn in to form a bicomposite structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

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BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to bicontinuous composites and more particularlyto bodies comprising two interconnected three dimensional networks. Inone particular aspect of the invention one network is a ceramic foam andthe other solidified material typically metal or plastics.

According to the invention in one aspect there is provided a method offorming a body comprising two interconnected three dimensional networks,the method comprising locating in a mold a preformed porous ceramicnetwork comprising an inter-connected skeleton having a density lessthan about 60% theoretical density and having pores the majority ofwhich range from about 20 to about 2000 micron, drawing molten metal orpolymeric material into the pores of the preformed porous ceramicnetwork and allowing the metal or polymeric material to solidifytherein.

Preferably the method includes the step of allowing the metal tosolidify in the pores at a low cooling rate to put the two networks inintimate contact. Preferably the molten metal or polymeric material isdrawn into the preformed porous until the ceramic skeleton is filledcompletely.

Preferably the molten metal or polymeric material is drawn into thepreformed porous ceramic by vacuum and/or pressure or the casting may bedone using a pressureless metal infiltration technique. The method maybe performed by squeeze casting.

In a preferred step the mould is preheated.

BRIEF SUMMARY OF THE INVENTION

Preferably the preformed porous ceramic phase has a controlled degree ofreticulation. The reticulation should be high to reduce the pressuregradient generated in metal infiltration and to minimise the level ofdefects associated with differential thermal contraction on cooling.Such defects can be shrinkage and interfacial debonding.

The method may include the subsequent step of removing all or part ofthe solidified metal or plastics.

Solidification of the polymeric material or resin may involve anexothermic reaction. Subsequent cooling is controlled to reduce thepotential for bond defects. Monomers may be polymerised in situ. Crosslinking agents may be used in modifying the properties of the polymericmaterial to optimise the properties of the composites.

In another aspect the invention provides a body comprising a preformedporous ceramic network having pores the majority of which range fromabout 20 to about 2000 micron, and a theoretical density less than about60% of theoretical density the pores being filled with solidified metalor plastics.

The foam ceramic has a substantially totally interconnected porosity atdensities less than 60%, preferably less than 30%, of theoreticaldensity. The density may range from about 10% to about 30%. Typicalaverage pore sizes are in the range 60-1400, preferably 60-650 micron.

The density of the foam ceramic is preferably below 30% to ensuresubstantially totally interconnected porosity. Higher densities may beuseful in circumstances in which a denser material is required forreasons of strength or permeability.

These denser materials, i.e. density higher than 30% of theoreticaldensity but in the case of the foaming technique based on agitationlimited to a maximum of 60% may be applied to the less dense materialsin the green or fired state to create porosity gradients across theporous article. Before use, the article will need to be fired. Thethickness of these layers may be varied to suit the application.

Higher density layers up to fully dense may be applied to the foamceramic in the green state by processing techniques such as gel casting,coagulation casting. The article formed will need to be fired beforeuse.

The proportions of the two phases may readily be adjusted so that thefoam ceramic makes up the major component by volume of the formed bodyor the metal phase does so.

The totally interconnected pore structure allows deep penetration of thepores of the foam ceramic. The penetrating material may form a separatecontinuous matrix or it may be simply deposited on the interior walls.

The ceramic foam may be made from particles such as oxides andnon-oxides. These materials are inherently stable to water or have asurface coating which is stable to the process conditions. Materialswhich have been used include alumina, zircon, spinel, silicon carbide,tin oxide, NZP, hydroxyapatite, zirconia, kyanite, cordierite and thelike.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1 and 2 show two exemplary microstructures in accordance with thesubject invention.

DETAILED DESCRIPTION OF THE INVENTION

It is much preferred of the invention that the skeleton is made by amethod according to our earlier patents.

In particular, in our patent EP0598783B we have described and claimed:“a method of making a porous refractory article composed of refractoryparticles, the method comprising the steps of:

-   -   a) forming a dispersion comprising particles in a liquid        carrier;    -   b) introducing gas into the dispersion;    -   c) removing the liquid carrier to provide a solid article having        pores derived from the bubbles;    -   d) drying; and    -   e) firing        characterised in that the dispersion contains a polymerisable        monomeric material.”

In our patent application WO98/15505 we have described and claimed: “amethod of melting a porous article composed of bonded particles (such ashydroxyapatite or the like) the method comprising the steps of:

-   -   a) forming a dispersion comprising a liquid carrier and the        particles and a polymerisable monomeric material;    -   b) forming a foam of the dispersion;    -   c) polynmerising the foamed structure;    -   d) drying the structure to remove the liquid carrier and provide        a solid article having pores derived from the bubbles, and    -   e) firing the article to remove the organic binder and provide a        ceramic bond        characterised in that small bubbles of gas are introduced in the        dispersion with agitation to form the foam and are allowed to        cause to coalesce before the polymerisation of the monomeric        material.

In our patent application GB 0009731.1 we have described and claimed amethod of making a ceramic foam by extrusion under low pressure. Thefoam ceramics made by that method are useful in the present invention.

It is intended that the entire disclosures of these earlier applicationsbe incorporated herein merely by these references.

The surface properties of the walls of the pores of the ceramic may bemodified to enhance the infiltration of the polymeric material or resinand/or control the bonding of the polymeric material or resin with theceramic. Some of the agents may also modify the surface properties ofthe walls. This can be done by:

-   i) impregnating the surface with solutions containing metallo    organic or inorganic salts. Various techniques of impregnation may    be used such as incipient wetness, simple impregnation, vacuum    impregnation, impregnation/deposition, etc. to establish the    required surface concentration of the inorganic/metallo-organic    salts. Promoters may be present, i.e. a material or treatment that    promotes hardening of a hydrated precursor to enhance the calcium    phosphate conversation. The surface concentration of calcium ions in    a porous hydroxyapatite article may be enhanced by impregnation of    the porous article with a solution containing calcium ions, drying    and heating to an elevated temperature if required or by    incorporating a calcium salt within the original composition of the    porous article. Such a modification allows the enhanced adsorption    of materials such as phosphonic acid esters. This can be done to the    preformed carrier as an after treatment or to the particles still to    be bonded together into the shape of an article, or-   ii) by firing the carrier to a temperature below that required for    full sintering of the ceramic. This value will depend on the nature    of the material of which the carrier is made.-   iii) by treating the surface with an acid or alkali, e.g. nitric    acid, phosphoric acid, caustic, selected according to the material    of the carrier.

The use of a degradable intermediate carrier is attractive because it isso versatile thus the deposit may be layered in different ways, e.g:

-   -   alternating layers of agent-free resin or polymer and of agent        containing layers;    -   varying the concentration of agent across different layers of        resin or polymer.

The metal phase may be provided by any suitable metal or alloy. Examplesinclude aluminium, aluminium alloys, ferrous alloys, copper alloy,magnesium alloy, and the like. Preferably the metal or alloy is selectedto have a melting temperature which will allow infiltration. Also, themetal or alloy should be compatible with the material of which theceramic is made. The metal phase may be in one or two layers. Thepolymer or resin may be biodegradable.

The polymer or resin phase may be provided by any suitable polymer orresin. Examples include polyethylenes, acrylates, methacrylates,polyesters, polyanhydrides, and the like. The viscosity of the polymeror resin is selected to allow facile penetration of the preformedceramic. Also, the polymer or resin should be compatible with thematerial of which the ceramic is made. The polymer or resin phase may bein one or two multi layers.

In the case of the plastics material the infiltration depth depends onthe gel-time of the resin. The infiltration depth also depends on thesealing of the outside of the body to be filled. The infiltration depthdepends on the viscosity of the resin used for infiltration. Theviscosity may be adjusted where necessary using additives.

The totally interconnected pore structure allows the penetration of thepores of the foam ceramic such that the penetrating material forms aseparate continuous matrix. The overall structure, i.e. the foam ceramicand the material in the pores, can be classified as bicontinuousmatrices. This allows the combination of the properties of the foamceramic and the penetrating material to be additive when, for instance,used as a composite. Alternatively the foam ceramic is used as atemporary carrier for the penetrating material which may be removed byheating, leaching, washing at the appropriate time to suit a particularapplication.

In a variation, metal is cast about and in a foam ceramic, followingwhich the ceramic skeleton is removed by acid or caustic treatment orleaching. A preformed porous ceramic foam of this invention isparticularly useful in this context. A turbine blade is an example of anarticle which may be made by this method.

In a preferred aspect of the invention the foam ceramic is infiltratedwith molten metal or metal alloys to form bicontinuous metal matrixcomposites. Vacuum and/or pressure may be used. The choice of metal ormetal alloy and ceramic can be optimised to suit a specific application.For instance, a preformed porous ceramic comprising foam alumina can beinfiltrated by aluminium alloy by techniques such as squeeze casting andother techniques known to those skilled in the art, to give abicontinuous aluminium metal matrix composite. The bicontinuous MMC hasmany advantages over MMCs with random distribution of the ceramicreinforcing phase such as increased wear resistance, increasedstiffness, enhanced thermal and electrical conductivity and good damagetolerance. The article is of low density, say about 2.8 to about 3g.cm³.

A body of the invention may be used in different applications. Oneparticular example is as disc brakes. The working environment of brakingdiscs in transport applications requires the materials to have highstiffness, good wear resistance, high thermal conductivity, good damagetolerance and low density. The combination of physical and mechanicalproperties of bicontinuous Al-MMCs satisfies those requirements verywell.

The additional advantages of bicontinuous Al-MMCs as materials for brakedisc applications compared with currently used cast iron discs are:

-   -   much less prone to squealing;    -   much less prone to thermal cracking;    -   possibilities of local reinforcement.

The porous ceramic can be shaped such that inserts of dense ceramic suchas alumina or perhaps metals can be put into place where perhaps ahigher mechanical strength is required from the implant in ultimatelyload bearing situations. The ceramic or metal insert may be exposed atone or more of the foam ceramic external surfaces or be enclosed withina shell of foam ceramic. The thickness of this shell may typically befrom 1 mm to 10 mm but is not limited to this range. Alternatively, thefoam ceramic may be the insert contained within a dense ceramic ormetal.

The method may be used to form articles useful in the automotiveindustry, e.g. crankshafts, water jackets, inlet manifolds, carburetorparts, convertible hard tops, roofs; aerospace components such aspropeller blades; sports frames, etc. for cycles, rackets, boats;building components such as bricks, wall plates, ceiling tiles.

The following list gives some examples in different fields of industry.

Building Industry:

-   -   sandwich elements for fire doors    -   cladding panels for houses    -   decking for oilrigs    -   fire protection tiles        Automotive and Transport Industry:    -   body panels    -   seat shells    -   heat shields    -   pump housings for oil pumps    -   interior panels and doors    -   aircraft trim panels    -   graphitized as carbon/carbon brakes for aircraft    -   automotive drive shafts, transmission housings, piston cylinder        rings; rail brake systems; turbine blades and vanes; cycle        frames.        Other:    -   in communications—satellite housings,    -   in sports equipment—golf club shafts, tennis racquet shafts.    -   in electrical equipment—turntables.    -   tank lining    -   military components    -   overwinding of epoxy pressure-pipes for fire protection    -   Bioceramics—replacement joints

The material may instead be selected from a wide variety of materials,such as general chemicals, petroleum derivatives, explosives, etc. Thefoam ceramic network holds these materials in a rigid matrix and soprotects them from mechanical stress or the like. The penetratingmaterial may also be a resin. For example, the foam ceramic matrix maybe impregnated with resins, polymers or lubricants in intimate contactwith the ceramic matrix. The choice of penetrating material and ceramiccan be optimised to suit the final application in a wide range ofindustries whether in lightweight structures, abrasive shapes,self-lubricating ceramic bearings.

In order that the invention may be well understood it will now bedescribed by way of illustration only with reference to the followingexamples.

EXAMPLE I

A slurry was mixed comprising 150 g alumina powder, 42 g of a 30 wt %solution of ammonium acrylate w/w also containing 1.25 wt % methylenebis acrylamide and 2 g water. To this was added 6 g Versicol KA11 andfurther mixed well for 5 minutes. 2 g of Dispex A40 was added and thiswas mixed further to form a low viscosity slip.

This was placed in a glove box and 10 drops of Tergitol TMN10 surfactantwas added. This was foamed using mechanical frothing to a set volume forthe density required.

Equal amounts of a 33% ammonium persulfate solution w/w andtetramethylethylene diamine were added. These were varied between 50 μland 30 μl according to the gelation time required which in turn affectsthe final pore size. The longer the gelation time the larger the pores.

Samples were removed from the beakers and left to dry in air for 48hours after which they were dried in a domestic microwave on low power.

The samples were fired at 1550° C. for 2 hours.

Three samples were made to have a nominal relative density of 10% oftheoretical, 20% of theoretical and 25% of theoretical.

EXAMPLE II

Two samples of cordierite ceramic foam with different pore sizes wereinfiltrated with an aluminium alloy, namely LM6, the composition ofwhich is given in Table 1. As the density of the foams was not measuredbefore infiltration, the samples will be referred to as 1 and 2, 1 beingmore a porous structure than 2. Each has a theoretical density of about20%. Infiltration was carried out by placing the ceramic preform in amould with four heaters surrounding the length of the mould to ensurethe metal remained liquid until the filling was complete. At the timethese samples were made the fourth heater (that furthest from the fillend) was not working.

TABLE 1 Composition of LM6 (maximum Wt. %) Al Si Cu Ni Fe Mg Mn Ti Zn PbSn Bal. 13 0.1 0.1 0.6 0.1 0.5 0.2 0.1 0.1 0.05

The samples were first cut in half to see how far the aluminium hadpenetrated through the foam and the quality of the fill. The crosssections were ground and polished to 1 μm diamond for macroscopicanalysis, and cubes of about 1 cm² were cut, mounted in conductingBakelite, ground to 1200 grit and polished to 1 μm diamond forexamination by optical and scanning electron microscopy. The opticalmicroscopy was carried out on an MEF3 microscope using both macro andbright field techniques, and a Cambridge Instruments Stereoscan 360 withEDX was used for examination by SEM. Through these analysis techniquesthe quality of the microstructure could be determined, in particularsigns of non-wetting of the ceramic by the metal, the quality of theinterface between metal and ceramic, and porosity after were looked at.Finally, small cubes were cut from the sample, and through measurementof both dimension and weight the densities of the infiltrated foams werecalculated.

The microstructure is shown in FIGS. 1 and 2. (500 magnification).

It will be seen that the Al foam fills and solidifies within the ceramicfoam. The aluminium has completely surrounded the foam, which isobserved as the grey granular structure. At the interfaces between thealuminium and ceramic there are also several small, different colouredparticles. These were characterised in the SEM and found to be silicon,α(AlFeMnSi), or β(AlFeSi) phases (compositions in Table 2), althoughother phases may also be present in quantities too small to analyse inthese samples. If we consider that the aluminium alloy probablysolidifies by a process of constitutional supercooling, then it isexpected that these intermetallic phases are the last to form whensufficient amounts of the minor elements have been rejected from thealuminium melt and concentrated at the edges form precipitates.

TABLE 2 Average composition of phases (Wt. %) Phase Al Si Cu Ni Fe Mg MnTi Other Silicon 0.5 98 0.5 0.5 0.5 (AlFeMnSi) 59 9.8 1.5 1.75 20 6.9(AlFeSi) 53 15 1.5 27 2.5

Small pieces from the infiltrated region were cut to measure therelative densities. The small cubes were measured in dimension (volume)by a micrometer and by weight on scales. The following calculations werethen made;

$\frac{{mass}\mspace{14mu}(g)}{{volume}\mspace{14mu}\left( {cm}^{3} \right)} = {{density}\mspace{14mu}\left( {g\text{/}{cm}^{3}} \right)}$For material 1;

${Density} = {\frac{0.513}{5.245 \times 6.420 \times 5.945 \times 10^{- 3}} = {2.56\mspace{14mu} g\text{/}{cm}^{3}}}$For material 2;

${Density} = {\frac{0.891}{8.920 \times 6.420 \times 5.860 \times 10^{- 3}} = {2.66\mspace{14mu} g\text{/}{cm}^{3}}}$

These results show a small difference in the densities of the twosamples. The density of LM6 is approximately 2.65 g/cm³ and Cordieriteis 2.5 g/cm³.

1. A method of forming a body comprising two interconnected threedimensional networks, the method comprising forming a porous ceramicnetwork comprising an inter-connected skeleton by: forming a dispersioncomprising ceramic particles in liquid carrier and a polymerizablemonomeric material; introducing gas into the dispersion; removing theliquid carrier to provide a solid article having pores derived from thegas; drying the solid article; and firing the solid article to providethe porous ceramic network; the porous ceramic network having a densityless than about 30% theoretical density, the majority of the poreshaving a range from about 20 to about 2000 microns; locating the porousceramic network in a mould, drawing molten metal or polymeric materialinto the pores of the preformed porous ceramic network and allowing themetal or polymeric material to solidify therein.
 2. A method accordingto claim 1, wherein the molten metal or polymeric material is drawn intothe preformed porous ceramic network by vacuum and/or pressure.
 3. Amethod according to claim 1, wherein the mould is preheated.
 4. A methodaccording to claim 1, wherein the molten metal or polymeric material isdrawn into the pores of the porous ceramic network using a pressurelessprocess.
 5. A method according to claim 1, comprising performing themethod by squeeze casting.
 6. A method according to claim 1, wherein thepreformed porous ceramic network has a density of 10 to 30% oftheoretical and average pore size of 60 to 1400 microns.
 7. A methodaccording to claim 1, including the subsequent step of removing all orpart of the solidified metal or polymeric material.